Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

Подождите немного. Документ загружается.

0031 Ascorbic acid has a number of chemical properties

responsible for the biological reactions that charac-

terize vitamin C. Usually, these characteristics are

positively associated with the benefits provided by

this vitamin. Paradoxically, these properties provide

difficulties in the analysis of ascorbic acid in foods

and physiological samples.

See also: Antioxidants: Natural Antioxidants; Browning:

Nonenzymatic; Cancer: Diet in Cancer Prevention;

Chromatography: High-performance Liquid

Chromatography; Controlled-atmosphere Storage:

Effects on Fruit and Vegetables; Food Fortification;

Oxalates; Oxidation of Food Components; Storage

Stability: Mechanisms of Degradation; Parameters

Affecting Storage Stability; Tocopherols: Properties and

Determination

Further Reading

Behrens WA and Made

re R (1994) Ascorbic acid, isoascor-

bic acid, dehydroascorbic acid and dehydroisoascorbic

acid in selected food products. Journal of Food Compos-

ition and Analysis 7: 158–170.

Budavari S (1996) The Merck Index, 12th edn. Whitehouse

Station, NJ: Merck.

Hodiroglou N, Made

re R and Behrens W (1998) Electro-

chemical determination of ascorbic acid and isoascorbic

acid in ground meat and in processed foods by high

pressure liquid chromatography. Journal of Food Com-

position and Analysis 11: 89–96.

Kall MA and Andersen C (1999) Improved method for

simultaneous determination of ascorbic acid and

dehydroascorbic acid, isoascorbic acid and dehydroi-

soascorbic acid in food and biological samples. Journal

of Chromatography B 730: 101–111.

Lykkesfeldt J (2000) Determination of ascorbic acid and

dehydroascorbic acid in biological samples by high-

performance liquid chromatography using subtraction

method: Reliable reduction with Tris[2-carboxyethyl]-

phosphine hydrochloride. Analytical Biochemistry 282:

89–93.

Martell AE and Hancock RD (1996) Metal complexes in

aqueous solutions. In: Fackler JP (ed.) Modern Inorganic

Chemistry. New York: Plenum Press.

Martell AE and Smith RM (1977) Critical Stability Con-

stants, vol. 3. New York: Plenum Press.

Packer L and Fuchs J (eds) (1997) Vitamin C in Health and

Disease. New York: Marcel Dekker.

Saxholt E and M

Øller A (1996) The Composition of Food,

4th edn. Copenhagen: Danish Veterinary and Food

Administration, Gyldendal.

Speek AJ, Schrijver J and Schreurs WHP (1984) Fluoro-

metric determination of total vitamin C and total

isovitamin C in foodstuffs and beverages by

high-performance liquid chromatography with precol-

umn derivatization. Journal of Agricultural and Food

Chemistry 32: 352–355.

Vanderslice JT and Higgs DJ (1991) Vitamin C content of

foods: sample variability. American Journal of Clinical

Nutrition 51: 1323S–1327S.

Vanderslice JT, Higgs DJ, Hayes JM and Block G (1990)

Ascorbic acid and dehydroascorbic acid content of food

as eaten. Journal of Food Composition and Analysis 3:

105–118.

Physiology

G F M Ball, Wembley, UK

Background

0001Ascorbic acid (vitamin C) has many diverse functions

as a reducing agent in the body. It keeps the bound

iron or copper ions of several hydroxylating enzymes

in the necessary reduced state, and aids the intestinal

absorption of iron by forming soluble ferrous–ascor-

bate complexes. Vitamin C helps in the fight against

cancer by reducing harmful free radicals to harmless

nonradical species, and inhibiting the formation of

Time (min)

123456789

Fluorescence response (mV)

DHIAA

DHAA

fig0006 Figure 6 Separation of DHAA and DHIAA on a C18 column

followed by postcolumn derivatization and fluorometric detec-

tion. From Kall MA and Andersen C (1999) Improved method for

simultaneous determination of ascorbic acid and dehydroascor-

bic acid, isoascorbic acid and dehydroisoascorbic acid in food

and biological samples. Journal of Chromatography B 730:

101–111, with permission.

324 ASCORBIC ACID/Physiology

carcinogenic N-nitroso compounds in the stomach.

There is increasing evidence that vitamin C is in-

volved in the human immune system and in the regu-

lation of prostaglandin synthesis.

Metabolism

0002 Humans and other primates, guinea-pigs, and fruit-

eating bats lack the enzyme gulonolactone oxidase,

which catalyzes the final step in the biosynthesis of

ascorbic acid, and thus rely on their diet to provide

the vitamin. Other animal species can synthesize as-

corbic acid from glucose and have no need for dietary

vitamin C.

Intestinal Absorption

0003 The overall system of intestinal transport and metab-

olism in humans is shown in Figure 1. Ascorbic acid is

ionized within the pH range of the intestinal contents,

and therefore, it is the ascorbate anion (specifically

l-ascorbate) that is transported. At physiological

intakes, ascorbate must move across the brush-border

membrane of the intestinal epithelium from a region

of lower concentration in the lumen to one of higher

concentration in the cytoplasm of the absorptive cell

(enterocyte). Such ‘uphill’ movement requires active

transport – a mechanism that depends ultimately

upon the expenditure of metabolic energy, i.e., the

energy released from the hydrolysis of adenosine tri-

phosphate produced during cellular metabolism. The

precise mechanism of ascorbate absorption is second-

ary active transport in which a transmembrane pro-

tein (inappropriately called a carrier) mediates the

sodium-coupled transfer of ascorbate across the

brush-border membrane. The carrier spans the mem-

brane in a weaving fashion and effects solute transfer

through a conformational change in its molecular

structure. The immediate energy source for the

transport mechanism is the concentration gradient

of sodium across the brush-border membrane. The

gradient is maintained by the constant extrusion

of sodium from the enterocyte by the action of the

sodium pump at the basolateral membrane. The

sodium pump is driven by metabolic energy and is

the primary driving force for ascorbate absorption.

0004An electrical potential difference ranging from 30

to 90 mV (cell interior negative) typically exists

across cell membranes. This membrane potential in-

fluences the active transport of charged solutes across

membranes. Using isolated brush-border membranes

from the intestines of guinea-pigs, it has been shown

that ascorbate transport is unaffected by changes in

the membrane potential. The transport system is

therefore electroneutral, indicating a 1:1 cotransport

of Na

and ascorbate



by the same carrier. l-

Ascorbic acid leaves the enterocyte by sodium-

independent facilitated diffusion at the basolateral

membrane.

0005Dehydro-l-ascorbic acid is taken up from the

bloodstream across the basolateral membrane of

enterocytes as well as being absorbed across the

brush-border membrane. The transport mechanism

in both cases is sodium-independent facilitated diffu-

sion driven by the steep concentration gradient

maintained by the intracellular reduction of dehy-

droascorbic acid.

0006The efficiency of vitamin C absorption of foods

over a range of usual intakes (20–120 mg per day)

is approximately 90%. Absorption by the sodium-

coupled secondary active transport mechanism

reaches its maximum rate at a relatively low luminal

concentration. The higher concentrations resulting

from ingestion of vitamin C supplements are absorbed

additionally by passive diffusion, which proceeds at a

very low rate. Absorption therefore becomes progres-

sively less efficient with increasing dose levels, the

efficiency falling from 50% of a single 1.5-g dose to

16% of a single 12-g dose. Absorption efficiency is

increased by the ingestion of several spaced doses

throughout the day, rather than the ingestion of a

single megadose. Ingesting the vitamin in a sustained

release form also improves absorption efficiency. As-

corbic acid bioavailability from fruits and vegetables

is not significantly different to that from synthetic

ascorbic acid, indicating that the bioavailability in

these natural food sources is high.

0007Intestinal absorption of ascorbate is adaptively

regulated in a transient and reversible manner by

the level of dietary ascorbate. The mechanism of

regulation is an increase or decrease in the number

of carriers at both brush-border and basolateral mem-

branes in response to low or high concentrations of

ascorbate in the blood. The rationale for adaptive

Brush-border membrane

of microvillus

Enterocyte

Basolateral membrane

DHAA

−

Connective

tissue

[H]

Intestinal

lumen

fig0001 Figure 1 Model of intestinal transport of the L-ascorbate anion

(AA



) and uncharged dehydro-L-ascorbic acid (DHAA) in vitamin

C-dependent animals. Thick arrowed lines indicate directional

pathways; [H] signifies enzymatic reduction.

ASCORBIC ACID/Physiology 325

regulation is that carriers are most needed at low

dietary ascorbate levels; at excessive levels, the re-

quired amount of ascorbate can be absorbed by

fewer carriers, aided by passive diffusion. As ascor-

bate does not provide metabolizable energy, there is

nothing to gain from the cost of synthesizing and

maintaining carriers when the vitamin supply is in

excess.

Postabsorptive Metabolic Events

0008 Vitamin C circulates in the bloodstream mainly as

free (nonprotein-bound) ascorbic acid. The kidney

actively reabsorbs ascorbate present in the glomerular

filtrate, thereby maximizing vitamin C conservation

in the body and helping the intestine to maintain the

circulating vitamin in its useful, reduced state. Con-

trary to the intestinal transport system, the renal

system is present also in species for which ascorbate

is not a vitamin (e.g., rat, rabbit). Renal uptake of the

l-ascorbate anion at the brush-border membrane of

the absorptive cell of the proximal convoluted tubule

is a sodium-coupled, secondary active transport

system. Unlike the corresponding intestinal transport

system, the renal system is affected by experimental

changes in the membrane potential, indicating an

electrogenic mechanism with a Na

/ascorbate



coupling ratio of 2:1. As the loaded carrier bears a

net positive charge, its transport is accelerated by the

negative membrane potential. Rapid renal reabsorp-

tion of ascorbate is important, as the transit time in

the proximal tubule is only about 10 s.

0009 Many cells and tissues can accumulate ascorbate

against a concentration gradient. Particularly high

concentrations of the vitamin are found in leukocytes

(white blood cells), lung tissue, the adrenal and pitu-

itary glands, and compartments of the eye. Skeletal

muscle contains much of the body’s pool of ascorbate,

although the concentration is relatively low. The ul-

timate metabolic fate of vitamin C is urinary excre-

tion of ascorbic acid or metabolites.

Effect upon Absorption of Inorganic Iron

0010 The iron present in natural foodstuffs exists in two

forms, heme iron and inorganic iron. Heme iron is

obtained from the hemoglobin and myoglobin pre-

sent in meat, fish, and poultry. Inorganic iron is found

primarily in plant foods; it provides 85–90% of iron

in a mixed diet and is the only source of iron in a

vegetarian diet. The mechanisms of absorption of

heme iron and inorganic iron differ. Unlike heme

iron absorption, absorption of inorganic iron is

affected by the presence of vitamin C and other con-

stituents of the diet.

0011Dietary inorganic iron is largely in the ferric (Fe

3þ

)

state, which is poorly soluble in the alkaline environ-

ment of the small intestine where absorption of iron

takes place. Ferrous iron (Fe

2þ

), however, is soluble at

alkaline pH. The amount of inorganic iron absorbed

by the intestine is influenced by stimulatory and

inhibitory factors of either dietary or endogenous

origin. Ascorbic acid both reduces and chelates iron,

forming ferrous–ascorbate complexes that are readily

absorbed. Other weak organic acids (e.g., citric acid),

certain sugars, and sulfur-containing amino acids

have similar properties, but are less effective. Meat,

fish, and poultry also enhance the absorption of inor-

ganic iron, apparently because of the binding of iron

by cysteine-containing peptides that result from the

digestion of muscle tissues. Phytates that occur in

cereals (especially in the bran fraction) bind inorganic

iron and retard its absorption. The same is true for

tannins (polyphenols) that are found in some vege-

tables and in tea.

0012Supplemental ascorbic acid consistently enhances

absorption of inorganic iron from single meals, as

indicated by incorporation of iron into red blood

cells. In one study, 500 mg of ascorbic acid taken

with the meal increased iron absorption about six-

fold. The most pronounced effects of ascorbic acid

are found in meals containing a high content of phy-

tates and/or tannins. This finding has led to the sug-

gestion that ascorbic acid enhances iron absorption

by counteracting the influence of inhibitory sub-

stances.

Biochemical Functions

0013Ascorbic acid acts as a cofactor for eight mammalian

enzymes (Table 1) by keeping enzyme-bound iron or

copper ions in the necessary reduced state. Other

reducing agents are far less effective as cofactors

tbl0001Table 1 Ascorbic acid-dependent mammalian enzymes

Enzyme Function

Prolyl-4-hydroxylase (EC 1.14.11.2) Collagen synthesis

Prolyl-3-hydroxylase (EC 1.14.11.7) Collagen synthesis

Lysyl hydroxylase (EC 1.14.11.4) Collagen synthesis

e-N-Trimethyllysine hydroxylase (EC

1.14.11.8)

Carnitine synthesis

g-Butyrobetaine hydroxylase (EC

1.14.11.1)

Carnitine synthesis

Dopamine b-hydroxylase (EC 1.14.17.1) Noradrenaline

synthesis

Peptidylglycine a-amidating

monooxygenase (EC 1.14.17.3)

Peptide amidation

4-Hydroxyphenylpyruvate hydroxylase

(EC 1.13.11.27)

Tyrosine metabolism

326 ASCORBIC ACID/Physiology

in vitro, and so these enzymes are considered to be

ascorbic acid-dependent. The roles of such enzymes

in four biochemical processes are discussed below.

Biosynthesis of Collagen

0014 Collagen is the major macromolecule of most con-

nective tissues. It forms a triple helix and is highly

cross-linked to give a rigid and inextensible structure.

The amino acid composition is unusual among

animal proteins, with an abundance of proline and

4-hydroxyproline and a few residues of 3-hydroxy-

proline and hydroxylysine. The hydroxyproline resi-

dues are necessary for proper structural conformation

and stability; hydroxylysine residues take part in

cross-linking and facilitate subsequent glycosylation

and phosphorylation.

0015 Free hydroxyproline and hydroxylysine are not in-

corporated into procollagen during its synthesis. The

presence of these two unusual amino acids in pro-

collagen (the precursor of collagen that contains

additional ‘propeptides’) arises through the post-

translational hydroxylation of particular proline

and lysine residues in the polypeptide chain. Within

the cisternae of the rough endoplasmic reticulum, the

newly synthesized procollagen chains encounter three

hydroxylating enzymes. Two of these enzymes, pro-

lyl-4-hydroxylase and prolyl-3-hydroxylase, convert

proline residues to 4-hydroxyproline or 3-hydroxy-

proline, respectively, and the third, lysyl hydroxylase,

converts lysine residues to hydroxylysine. Each of

these enzymes contains an iron ion (maintained in

the ferrous state by ascorbate) and requires molecular

oxygen and a-ketoglutarate as cosubstrates (Figure 2).

In addition to the posttranslational modifications of

procollagen, ascorbate stimulates transcription of the

type I procollagen gene. The absence of wound

healing is one of the features of scurvy that can be

attributed to impaired collagen synthesis arising from

lack of vitamin C.

Biosynthesis of Carnitine

0016Carnitine (3-hydroxy-4-N-trimethylaminobutyric acid,

Figure 3) is essential for the b-oxidation of long-chain

fatty acids, producing energy in the mitochondria.

Neither free fatty acids nor fatty acid coenzyme As

can penetrate the inner membranes of mitochondria,

but acylcarnitine can readily do so. The translocation

of fatty acids from the cytosol to the b -oxidation site

in the matrix of the mitochondrion is therefore

dependent upon carnitine.

0017The synthesis of carnitine commences with the

conversion of specific peptide-linked lysine residues

to trimethyllysine within certain proteins. Proteins in

which trimethyllysine has been found include his-

tones, myosin, calmodulin, and cytochrome c. Free

trimethyllysine released by proteolytic cleavage is

converted to carnitine in a number of steps, which

include two hydroxylations. The enzymes responsible

for these hydroxylations are e-N-trimethyllysine

hydroxylase and g-butyrobetaine hydroxylase. Like

the hydroxylases involved in collagen biosynthesis,

both enzymes contain ferrous ion and require molecu-

lar oxygen and a-ketoglutarate as cosubstrates.

0018Vitamin C deficiency results in a variable decrease

in carnitine levels of skeletal muscle, heart muscle,

liver, and kidney. Carnitine status depends on dietary

carnitine intake, a modest rate of carnitine biosyn-

thesis, and efficient conservation of carnitine by renal

reabsorption. The rate of carnitine synthesis in

scorbutic guinea-pigs is much higher than the rate

COOH

Prolyl-4-hydroxylase

, ascorbate

Peptide-bound

proline

Peptide-bound

hydroxyproline

α-Ketoglutarate

Succinate

fig0002 Figure 2 Hydroxylation of proline.

CH CH

COO

−

fig0003 Figure 3 L-Carnitine.

ASCORBIC ACID/Physiology 327

in normal guinea-pigs, and therefore impaired syn-

thesis is not responsible for carnitine depletion. The

responsible factor is, in fact, increased urinary excre-

tion resulting from inefficient renal reabsorption.

Biosynthesis of Noradrenaline

0019 As a neurotransmitter, noradrenaline (norepineph-

rine) is synthesized in the nerve terminals of post-

ganglionic sympathetic neurones, referred to as

adrenergic neurones. As a hormone, noradrenaline,

along with adrenaline, is synthesized in the chro-

maffin cells of the adrenal medulla. The final and

probably rate-limiting step in the biosynthetic path-

way – the conversion of dopamine to noradrenaline

(Figure 4) – is catalyzed by dopamine b-hydroxylase

in the presence of molecular oxygen. Dopamine

b-hydroxylase is a tetramer, each monomer contain-

ing two copper ions maintained in the cuprous (Cu

)

state by ascorbate. The enzyme is localized within

storage vesicles in both adrenergic neurones and adre-

nomedullary chromaffin cells. Intravesicular ascor-

bate is maintained in the reduced state by electron

transport across the vesicle membrane, cytosolic

ascorbate being the most likely electron donor. Cyto-

solic semidehydroascorbate generated in the extra- to

intravesicular electron transfer reaction may be re-

duced by the outer mitochondrial membrane enzyme,

semidehydroascorbate reductase (EC 1.6.5.4), to

complete the ascorbate regeneration cycle. In this

manner, a stable rate of noradrenaline biosynthesis

can be maintained in the absence of exogenous ascor-

bate.

Amidation of Peptides to Hormones and Hormone-

releasing Factors

0020A large number of peptides act as hormones and

hormone-releasing factors. The vast majority of

these peptides are initially synthesized as larger, in-

active precursor molecules, which are converted to

their active forms through a series of posttrans-

lational modifications. One such final step of acti-

vation is carboxy-terminal a-amidation, which is

essential for biological activity. Examples of

a-amidated peptides include a- and g-melanotropins,

calcitonin, pro-ACTH, vasopressin, oxytocin, chole-

cystokinin, gastrin, gastrin-releasing peptide, and

releasing factors for growth hormone, corticotropin,

and thyotropin. Direct precursors to a-amidated pep-

tides have glycine as the carboxy terminal amino acid.

The enzyme responsible for the amidation is the

bifunctional peptidylglycine a-amidating monooxy-

genase (PAM), which is found in secretory granules

in many neuroendocrine tissues. PAM is actually a

precursor protein that undergoes endoproteolytic

cleavage to generate two enzymes, peptidylglycine

a-hydroxylating monooxygenase (PHM) and pepti-

dyl-a-hydroxyglycine a-amidating lyase (PAL). The

two-step nature of the amidation is shown in Figure

5. Step 1 requires molecular oxygen, cuprous ion, and

ascorbate as the reductant.

Antioxidant Role

0021Ascorbic acid helps to fight diseases such as cancer by

scavenging harmful free radicals. A free radical can be

dopamine β-hydroxylase

, ascorbate

Dopamine Noradrenaline

fig0004 Figure 4 Hydroxylation of dopamine.

Step 1

Step 2

Peptidyl

Peptidylylycine Peptidyl-α-hydroxyglycine

OH Peptidyl NH CH C

OH O

HCC

OHPeptidyl NH Peptidyl NH

CH C

OH O

PHM

PAL

, ascorbate

α-Amidated peptide

Glyoxylate

fig0005 Figure 5 Amidation of peptidylglycine.

328 ASCORBIC ACID/Physiology

defined as an atom or molecule capable of an inde-

pendent existence that contains an unpaired electron

in its outer orbit. A superscript dot is used to denote

free radical species; the superoxide anion radical with

its two unpaired electrons is usually represented by



. Owing to their unstable electronic configuration,

free radicals are much more reactive than nonradi-

cals. They readily extract electrons from other mol-

ecules with which they collide, and these molecules in

turn become free radicals. Thus, a chain reaction is

propagated.

002 2 Free radicals can be generated anywhere within the

cell by a variety of processes: spontaneous autoxida-

tion reactions (e.g., flavin oxidation), phagocytosis

during infection, metabolic processing of foreign

compounds (e.g., constituents of tobacco smoke,

drugs, pesticides, solvents, and pollutants), ultravio-

let irradiation of the skin, and ionizing radiation.

Among free radicals of biological importance are

the hydroxyl (HO

), peroxyl (ROO

), superoxide



), and nitric oxide (NO

) radicals.

002 3 Many, or perhaps most, free radicals that are gen-

erated in the body are metabolized to nonradicals by

enzymes such as superoxide dismutase (EC 1.15.1.1),

which catalyzes the reduction of the superoxide rad-

ical to hydrogen peroxide and water (1).



þ O



þ 2H

! H

þ O

:ð1 Þ

002 4 Although hydrogen peroxide is relatively unreact-

ive, it can traverse biological membranes and cause

intracellular damage through the production of hy-

droxyl radicals catalyzed by ferrous or cuprous

ions (2).

2þ

ðor Cu

ÞþH

!Fe

3þ

ðor Cu

2þ

þ HO

þ OH



:ð2 Þ

002 5 Two enzymes remove hydrogen peroxide in mam-

malian cells: catalase (EC 1.11.1.6) (eqn (3)) and

glutathione peroxidase (EC 1.11.1.9) (eqn (4)),

whichuseshydrogenperoxidetooxidizereducedglu-

tathione(GSH)tooxidizedglutathione(GSSG).

! 2H

O þ O

ð3 Þ

þ 2GSH ! GSSG þ 2H

O :ð4 Þ

002 6 Enzymes do not completely prevent the formation

of free radicals, and various secondary antioxidants

fulfill the role of free-radical scavengers. Ascorbate is

one such antioxidant within the aqueous environ-

ment of the cytosol and extracellular fluids, and at

the water–lipid interface of cellular and subcellular

membranes. Figure 6 shows a possible scheme in

which ascorbate can be recycled after trapping a free

radical. Ascorbate (AH



) donates an electron to the

free radical (R

), forming detoxified product (R), and

is itself oxidized to the ascorbyl free radical (A



), also

known as semidehydroascorbate. Pairs of ascorbyl

free radicals disproportionate to form one molecule

each of dehydroascorbic acid (A) and ascorbate. The

high disproportion rate constant of the ascorbyl free

radical prevents substantial interaction with other

substances. Dehydroascorbic acid can be reduced to

ascorbate by either a glutathione-dependent or an

NADPH-dependent reductase.

0027In the lipid environment of cellular and subcellular

membranes and plasma lipoproteins, vitamin E (toco-

pherol, TOH) protects the lipids from peroxidation

by reacting with peroxyl radicals, forming the toco-

pheroxyl radical (TO

) and lipid hydroperoxide

(ROOH), thereby terminating the chain reaction

(eqn (5)). The potentially harmful lipid hydroperox-

ide can be converted enzymatically to a fatty acid

alcohol. Like the ascorbl radical, the tocopheroxyl

radical has a low reactivity and can be converted

back to tocopherol by enzyme systems.

ROO

þ TOH ! ROOH þ TO

:ð5Þ

0028In model in-vitro systems, ascorbate regenerates

vitamin E by reducing the tocopheroxyl radical back

to tocopherol, producing the ascorbyl radical (eqn

(6). However, any vitamin E sparing effect exerted

by vitamin C in vivo is negligible in comparison

with other metabolic processes.

þ AH



! TOH þ A



: ð6Þ

0029Ascorbate can exert pro-oxidant properties in vitro

through interaction with iron or copper ions and

−

(a)

(d)

(b)

(c)

fig0006Figure 6 Recycling of ascorbate during the process of free

radical scavenging. AH



, ascorbate; A

.

, ascorbyl free radical

(semidehydroascorbate); A, dehydroascorbic acid; R

, free rad-

ical species; R, detoxified species. (a) Single-electron transfer;

(b) semidehydroascorbate reductase; (c) nonenzymatic dismuta-

tion of ascorbyl free radical; (d) dehydroascorbic acid reductase.

ASCORBIC ACID/Physiology 329

hydrogen peroxide to produce hydroxyl radicals. In

the healthy human body, these metal ions are largely

sequestered in forms unable to catalyze free radical

reactions, and so the pro-oxidant properties of ascor-

bate would not normally be expected to be biologic-

ally significant.

Inhibition of

-nitroso Compound

Formation

003 0 Nitrosating agents, for example nitrous anhydride

), are derived from nitrite (eqns (7)–(9))



þ H

Ð HNO

ð7Þ

HNO

þ H

Ð H

ð8Þ

þ NO



Ð N

þ H

O :ð9Þ

0031 In the acidic contents of the stomach, nitrosation

reactions take place between nitrosating agents and

secondary amines or N-substituted amides to form N-

nitrosamines (eqn (10)) or N-nitrosamides, collect-

ively called N-nitroso compounds.

NH + N

3(10)

O + HNO

0032 N-Nitroso compounds are implicated in cancer of

the stomach, esophagus, and nasopharynx. Nitrite is

present naturally in some foods and is added as a

preservative in cured meats. Nitrosatable amines

and amides may be ingested as drugs, food additives,

or natural constituents of foods. Ascorbic acid is

added to the curing brine in the production of bacon

and other cured meats in order to prevent the subse-

quent formation of N-nitroso compounds in the

stomach. Presumably, the vitamin acts by reducing

nitrous anhydride to nitric oxide (eqn (11)), which is

not a nitrosating agent.



þ N

Ð A þ 2NO þ H

O :ð11Þ

0033 For ascorbic acid to be effective, it must be present

in sufficient quantity, since nitric oxide can be oxi-

dized to nitrogen dioxide, and these two species can

combine to form nitrous anhydride (eqn (12)).

NO þ NO

Ð N

:ð12Þ

Immune Function

003 4 Vitamin C possesses antihistamine activity and is im-

plicated in a variety of immune functions, including

neutrophil chemotaxis, T lymphocyte proliferation,

natural killer cell activity, and activation of the com-

plement component C1q. Neutrophils and lympho-

cytes (types of leukocytes) concentrate vitamin C

at levels up to 100 times higher than in plasma,

suggesting that the vitamin has a physiological role

in these immune-system cells.

Antihistamine Activity

0035In the initial stages of an immune response, histamine

amplifies immunoresponsiveness by increasing capil-

lary permeability and smooth muscle contraction,

enhancing the flow of immune factors to the site of

injury or infection. Subsequently, histamine sup-

presses the immune response by inhibiting neutrophil

lysosomal enzyme release. Excess histamine has a

negative effect, and antihistamine therapy is well

known for alleviating conditions such as hay fever

and inflammatory skin disorders. Ascorbic acid de-

grades histamine in vitro. In humans, chronic oral

administration of vitamin C results in a lowering of

blood histamine levels, whereas low blood levels of

ascorbate are associated with increased histamine

levels.

Neutrophil Chemotaxis

0036Neutrophils (polymorphonuclear leukocytes) destroy

pathogenic organisms by phagocytosis (engulfment)

and intracellular enzymatic degradation. If foreign

materials are too large to be engulfed, enzymes and

reactive oxygen species are released extracellularly.

Neutrophil chemotaxis refers to the response of

neutrophils to chemical signals produced at sites of

infection. Histamine lowers the chemotactic respon-

siveness of neutrophils by elevating intracellular

levels of cyclic adenosine 3

-monophosphate (cyclic

AMP). A rise in cyclic AMP is associated with de-

pressed neutrophil motility. Data from a study using

healthy human subjects suggest that vitamin C at

doses of 2 g per day for 2 weeks may indirectly en-

hance neutrophil chemotaxis through its antihista-

mine activity.

Proliferation of T Lymphocytes

0037Vitamin C regulates proliferation of T lymphocytes,

which are responsible for destroying viruses that

reside and multiply inside living cells of the body.

Specialized types of T lymphocyte exert immuno-

regulatory and effector functions through the secre-

tion of lymphokines such as interferon. Cytotoxic

T lymphocytes kill viruses by attaching to the infected

cell through recognition of viral antigens and releas-

ing granules whose contents cause perforation of the

host cell membrane. The granules also contain sub-

stances that induce apoptosis – genetically pro-

grammed cell death by fragmentation of nuclear

DNA. Since death by apoptosis does not result in

the release of the cell’s contents, killing by this mech-

anism may prevent the spread of infectious virus into

other cells.

330 ASCORBIC ACID/Physiology

Natural Killer Cell Activity

0038 Natural killer cells are large nonphagocytic leuko-

cytes that kill tumor cells by cell membrane perfor-

ation and apoptosis. The cytotoxic activity of killer

cells is spontaneous, unlike that of cytotoxic

T lymphocytes, which only develops after an immune

response has been initiated. An in-vivo effect of as-

corbic acid on enhancement of human natural killer

cell activity has been reported at a dosage of 60 mg

per kilogram of body weight.

Regulation of the Complement Component C1q

0039 Complement is a collective term for at least 25

plasma and cell membrane proteins that are import-

ant in the host’s immune system. Following initial

activation by antigen–antibody complexes, the vari-

ous complement components interact, in a highly

regulated enzymatic cascade, to generate various

reaction products. Some of these products induce

localized vasodilation and attract phagocytic cells

chemotactically. Other products coat the foreign

particle, enabling it to be recognized by phagocytic

cells that bear receptors for these complement prod-

ucts. Finally, the terminal components of the comple-

ment system generate the membrane–attack complex,

which lyses bacteria and membrane-enveloped

viruses.

0040 The first complement component to be activated in

the classical cascade pathway is the C1 complex, in

which C1q is one of three proteins. When guinea-pigs

were fed tissue-saturating amounts of vitamin C, C1q

concentrations were significantly higher than in those

animals fed only enough ascorbate for adequate

growth and for the prevention of scurvy. C1q is a

hydroxyproline-containing protein, and so the ob-

served effect is consistent with the known role of

vitamin C in hydroxyproline synthesis.

Prostaglandin Synthesis

0041 The prostaglandins (PG) are a group of hormone-like

lipids formed in the body from derivatives of essential

fatty acids, particularly arachidonic acid (20:4n-6).

Prostaglandins have a vast range of effects and

modulate cardiovascular, pulmonary, immune, and

reproductive functions. Vitamin C stimulates the for-

mation of PGE

from dihomo-g-linolenic acid in

human platelets. This effect occurs over the physio-

logical range of vitamin C concentrations and is de-

sirable because PGE

is an inhibitor of platelet

aggregation. PGE

is also required for T lymphocyte

formation, regulation of collagen and cholesterol me-

tabolism, and regulation of responsiveness to insulin.

There are sufficient data to suggest that some of

vitamin C’s biological actions could be explained by

a regulatory effect on prostaglandin synthesis.

Dietary Intake and Supplementation

Recommended Dietary Allowance (RDA)

0042In the USA, the 1989 RDA for vitamin C is 60 mg per

day for adults. At this level, saturation of tissue bind-

ing and maximal rates of metabolic and renal tubular

absorption seem to be approached. The RDA of

100 mg per day for smokers is higher because the

plasma concentration of vitamin C is lowered by the

use of cigarettes. Plasma ascorbate levels of women

decrease during pregnancy, and the RDA includes a

10 mg per day increment for pregnant women. A

daily increment of 35 mg is recommended during the

first 6 months of lactation and 30 mg thereafter. As-

sessments of human vitamin C status are based upon

measurement of ascorbate concentrations in plasma

and leukocytes.

Deficiency

0043Scurvy, the classical vitamin C deficiency disease, can

occur under circumstances of poor diet, such as arises

in chronic alcoholism.

Toxicity

0044Ascorbic acid is generally regarded as being nontoxic

at high intakes. Once the plasma concentration of

ascorbate reaches the renal threshold, it is excreted

more or less quantitatively with increasing intake.

Excessive daily intakes of vitamin C can cause an

increased production of oxalic acid in some individ-

uals, leading to an increased risk of kidney stone

formation.

Rebound Scurvy

0045Theoretically, the absorption of ascorbic acid could

be impaired on resumption of normal vitamin C

inputs following megadosing (> 1 g per day), because

of insufficient carriers in the enterocyte cell mem-

branes. Based on experiments with guinea-pigs, it is

considered likely that, in humans, renewed synthesis

of carriers will take place well before the onset of

scurvy. During megadosing, reduced ascorbate ab-

sorption is accompanied by increased rates of ascor-

bate catabolism. In adult guinea-pigs, the accelerated

catabolism is not reversible after more than 2 months

on subnormal uptake of ascorbate. Guinea-pigs are

thus susceptible to a systemic conditioning effect

known as rebound scurvy, caused by an induction of

ascorbic acid-metabolizing enzymes by high dietary

vitamin C. The body stores of vitamin C are depleted

more rapidly in juvenile guinea-pigs than in adults,

ASCORBIC ACID/Physiology 331

increasing the likelihood of rebound scurvy in juven-

iles. In human infants, rebound scurvy could possibly

occur, despite an adequate daily intake of vitamin C,

if the mother has received large supplements of the

vitamin during pregnancy. Reported cases of rebound

scurvy in adult humans are rare.

Treatment of the Common Cold

0046 Vitamin C supplementation has been suggested for

the prevention of the common cold and the allevi-

ation of its symptoms. Reviews of numerous studies

generally conclude that vitamin C megadoses have no

consistent effect on reducing the incidence of colds in

people in general. However, in four studies with Brit-

ish male schoolchildren and students, a statistically

high significant reduction in common cold incidence

was found in vitamin C-supplemented groups. This

could be interpreted as a correction of a marginal

deficiency in the study subjects, rather than an effect

of the high dosage. Placebo-controlled studies have

consistently shown that high doses of vitamin C alle-

viate common cold symptoms, although the validity

of some of these studies has been questioned.

See also: Adaptation – Nutritional Aspects;

Antioxidants: Natural Antioxidants; Bioavailability of

Nutrients; Cancer: Diet in Cancer Prevention; Dietary

Reference Values; Immunology of Food;

Nitrosamines; Prostaglandins and Leukotrienes;

Scurvy